1. Field of the Invention
The present invention relates to an optical system in which substantially the same images can be formed at two or more different positions by branching or switching an optical path from a single photographing optical system into predetermined number of optical paths, and in particular, relates to an optical system in which images can be formed at two or more different positions at different magnifications, and further to an optical system which can be used in a surveillance camera system and the like, called a composite camera, which are required to form images simultaneously on an image pick-up device and a silver halide film, which have differently sized image planes.
2. Description of the Related Art
As a typical conventional surveillance camera system, a single-lens-reflex type surveillance camera is well known in the art. In the above surveillance camera, a bundle of rays transmitted through a photographing optical system (the first image-forming optical system) is reflected by a mirror, and the image of an object is formed on an equivalent image plane; and further, through a relay optical system (the second image-forming optical system), the image formed on the equivalent image plane is re-imaged on an image pick-up device (CCD: a re-image forming plane).
In the above surveillance camera, a condenser lens group is provided behind the equivalent image plane so that the exit pupil of the photographing lens system and the entrance pupil of the relay optical system becomes optically conjugate (hereinafter, conjugate). In other words, the condenser lens group establishes the conjugate relationship between the pupils of the first and second image-forming optical systems.
However, in the case where a mechanical diaphragm with a variable aperture for controlling an exposure on the image plane of the first image-forming optical system is provided in the photographing optical system of a conventional composite camera, the smaller the variable aperture becomes, the more an eclipse occurs with respect to off-axis rays towards the relay optical system, and thereby peripheral illumination is extremely reduced. The optical system with such a problem is not suitable for practical use. In other words, the optical system of a conventional composite camera cannot adequately control an exposure.
It is an object of the present invention to provide an optical system for a composite camera (i) which includes a first image-forming optical system, and a second image-forming optical system for re-imaging an image, formed on an image plane equivalent to an image plane of the first image-forming optical system, onto a predetermined position, and (ii) in which no eclipse occurs with respect to off-axis rays towards the second image-forming optical system even when a mechanical diaphragm with a variable aperture is provided in the first image-forming optical system, and even when the aperture of the mechanical diaphragm is made smaller. In other words, the object of the present invention is to provide the optical system for the composite camera in which the mechanical diaphragm can be provided in the first image-forming optical system without causing any adverse effect on peripheral illumination around the re-image forming plane.
The conception of the present invention is based on the following factors which are the causes of the above problem:
(i) the exit pupil of the first image-forming optical system and the entrance pupil of the second image-forming optical system is conjugate only in the vicinity of the optical axis; and
(ii) when a diaphragm is provided in the first image-forming optical system, i.e., an effective aperture thereof is reduced, an eclipse occurs in the second image-forming optical system.
According to the present invention, there is provided an optical system of a composite camera including a first image-forming optical system, and a second image-forming optical system for re-imaging an image, formed on an image plane equivalent to an image plane of the first image-forming optical system (hereinafter, equivalent image plane), onto a predetermined position (hereinafter, re-imaging plane). The optical system of the composite camera further includes a condenser lens group, which includes two or more lens elements, or includes a single lens element on which at least one aspherical surface is formed, is provided between the equivalent image plane and the second image-forming optical system; and at any image height on the equivalent image plane, the following condition is satisfied:
xcex94xcex8Pmaxxe2x88x92xcex94xcex8Pmin less than 2xc2x0 . . .xe2x80x83xe2x80x83(1)
wherein
xcex94xcex8Pmax designates the maximum value of xcex94xcex8P(y)=xcex8BP(y)xe2x88x92xcex8AP(y);
xcex94xcex8Pmin designates the minimum value of xcex94xcex8P(y)=xcex8BP(y)xe2x88x92xcex8AP(y);
xcex8AP(y) designates an angle of incidence, at an image height y, of a principal ray which is transmitted through the first image-forming optical system, and is incident on the equivalent image plane (the image plane of the first image-forming optical system); and
xcex8BP(y) designates an angle of incidence, at the image height y on the equivalent image plane, of a principal ray to be incident on the re-imaging plane, through the first image-forming optical system, the condenser lens group and the second image-forming optical system.
By satisfying condition (1) at any image height on the equivalent image plane, the conjugate relationship between the exit pupil of the first image-forming optical system and the entrance pupil of the second image-forming optical system can be established not only in the vicinity of the optical axis but also over the entire pupil area. As a result, even if an effective aperture of the first image-forming optical system is reduced, no harmful eclipse occurs in the second image-forming optical system. In other words, a mechanical diaphragm with a variable aperture can be provided in the first image-forming optical system. Consequently, it is possible to provide a diaphragm in the second image-forming optical system as well.
More concretely, the first image-forming optical system is a photographing optical system with respect to a silver halide film; and the second image-forming optical system is a photographing optical system with respect to an image pick-up device.
Further, the present invention can be applied to an optical system in a single-lens-reflex type optical device in which the equivalent image plane is formed by a mirror which is positioned between the first image-forming optical system and the image plane thereof.
The present disclosure relates to subject matters contained in Japanese Patent Applications No. Hei-11-101552 (filed on Apr. 8, 1999) and Hei-11-101553 (filed on Apr. 8, 1999) which are expressly incorporated herein by reference in their entirety.